The present invention relates to a process for the concentration of an alkaline solution wherein an alkaline film falling downwardly along the inside of at least one evaporator tube is heated by a heat-transfer fluid conveyed upwardly along the outside of the tube.
Relatively high temperatures are required for the concentration of alkaline solutions which usually are obtained as a 50% solution and are to be concentrated to practically an anhydrous melt. Therefore, a salt melt has proven to be suitable as the heat-transfer fluid, which melt is fed at, for example, about 450.degree. C. to the lower portion of the heating jacket surrounding the evaporator tube and makes it possible at this location to reach, with a boiling temperature of the alkaline solution of 430.degree. C., a concentration of about 99%. If the alkaline solution which has a strength of, for example, 50% is fed at a temperature of about 140.degree. C. to the upper portion of the evaporator and the salt melt is discharged at that location from the heating jacket at a temperature, for example, of about 380.degree. C., a temperature difference of, for example, 20.degree. C. prevails between the boiling liquid and the salt melt in the lower portion of the falling-film evaporator tube, whereas a temperature difference of 240.degree. C. exists in the upper portion of the evaporator tube. This large temperature difference between the heat-transfer fluid and the boiling alkaline solution in the upper portion of the tube leads to a very high heat flow density (heat flux) and as a result, a complete lack of wetting of the evaporator surface takes place at certain spots. This is a well-known phenomenon (known as the Leidenfrost effect) if the heat flow density exceeds a certain magnitude on evaporator surfaces.
Under the above-described operating condition, a heat flow density of more than 250,000 kcal/hm.sup.2 evolves, and under these circumstances it is unavoidable for the evaporator surface to "run dry" at spots.
It is known that evaporator tubes made of pure nickel or high-nickel alloys have proven to be rather highly corrosion-resistant when used for the concentration of alkaline solutions to practically anhydrous melts. However, the prerequisite for this effect is that an uninterrupted nickel oxide film is formed on all surfaces exposed to the solution. It has been found that this is the case in a falling-film evaporator tube as long as the liquid film uniformly covers the evaporator surface. However, the aforementioned high heat flow densities which thus are critical regarding the lack of wetting of the evaporator surface, however, lead not only to a destruction of the nickel oxide film, but at the same time result in intermittent thermal stresses in the evaporator tube wall. These factors, in combination, can lead to a rapid destruction of the evaporator tubes.
Accordingly, it is an object of the present invention to provide a process of the aforementioned type and an apparatus for conducting said process wherein the above-described disadvantages are eliminated. The process of the present invention is characterized in that, in order to avoid critical heat flow densities on the evaporator surface, the flow rate of the heat-transfer fluid is reduced in the upper heating section of the heating jacket when compared to the sections lying in the lower portion thereof. This is achieved by providing a heat exchange apparatus wherein the heating jacket which coaxially surrounds the cylindrical evaporator tube is radially widened in its upper section leading to the fluid outlet. The flaring (enlarging) of the heating jacket section can be conical or cylindrical or otherwise. It is also possible to provide several cylindrical sections which increasingly widen in the upward direction, or to provide a cylindrical section which adjoins a conical portion. In any event, the flaring feature leads to a reduction of the flow rate of the heat-transfer fluid, so that it is readily possible by a suitable selection of the size of the flaring section to maintain a flow rate in the upper zone, which has a high temperature difference between the heat-transfer fluid and the alkaline solution, at such a low value that, with a correspondingly reduced heat-transfer coefficient, a subcritical heat flow density can be achieved. Thus a heat flow density can be obtained which does not lead to a lack of wetting. This advantageous result can be provided without undesirably affecting the conditions in the lower region of the heat-exchange system which has only minor temperature differences.